Lexus GS 300 engine surge & brake problem

lexus
brakes

#1

My 2000 Lexus GS 300 has done the following three times when I’ve been inching forward by lightening on the brake (not touching the gas pedal): slight engine surge moves the car forward and the brakes don’t work. I have to throw the car into “Park” to stop. Had it checked each time (twice by my local shop and once by the dealer) and they find nothing amiss. These three incidents have occurred several weeks apart, during cold weather. My car has 98,000 miles on it and has been very well maintained. Thoughts, anyone?


#2

@Jwo

This sounds like a vacuum problem, which would have a huge effect on your vacuum power brake booster

When the brakes “don’t work” . . . is the brake pedal rock hard, and stepping on it has no effect?

In my opinion, you have either a bad booster, check valve, or the booster hose

This should not be difficult for a competent mechanic to diagnose and repair. The mechanic should also check that the engine itself is producing sufficient vacuum at idle

By the way, a vacuum problem can affect your power brakes AND the engine idle

I said this because the title also mentions an “engine surge”


#3

Can you describe what you mean by “the brakes don’t work”? Do you mean that the pedal goes to the floor? Or do you mean that the pedal feels really stiff and hard to push down?

I’m making a WAG that it is the latter and I would replace the brake booster vacuum line and check valve. You might be headed for a new brake booster.


#4

Your booster check valve and/or your booster is bad.

The way the system works, vacuum from the engine creates vacuum on the forward surface of a diaphragm in the booster canister. That vacuum pulls on the diaphragm and assists you in pressing the brake rod into the master cylinder. When you let the brakes up, the center rod moves a valve and the booster chambers become vented to ambient. The check valve is designed to allow vacuum from the engine to affect the diaphragm, but prevent vacuum created in the booster’s forward chamber when you let the pedal up from affecting the engine’s intake.

I suspect that what’s happening in your case is that the booster and perhaps also the check valve is malfunctioning, allowing the pressure drop in the booster’s forward chamber when you back off the pedal to be felt by the engine and not allowing the chambers to vent, creating a compressed air chamber in front of the diaphragm that’s preventing you from applying the brakes again until that pressure bleeds off.

Try a quick test. With the engine off, pump the brakes. The pedal should rise and become hard. Then, while pressing on the pedal, start the engine. The pedal should sink and soften. Failure of this test would pretty much diagnose a bad booster. If it passes, it could still be the booster, but the problem could be intermittent.

Post back with the results.


#5

Thank you for the feedback and ideas. My mechanic said that the brakes, vacuum, valves, booster etc. were checked. He did “relearn” and clean the throttle and adjust the idle. I’ve been wondering when they say that they’ve checked something, or run diagnostics, does that mean they actually examine the part? Or just hook things up to a computer?

Regarding the feel of the brakes during the incidents; the first time the pedal suddenly went down further and the car stopped; the second two times I had no response to pressing the brake but I honestly can’t say if the pedal felt stiff any of those times. I will try that brake test Mountainbike mentions.

I know zip about cars and I don’t know how to frame my questions in order to press for the right information when talking to the repair shop (which has given me great service over the 12 years I’ve had my car). …I may just send them a link to this thread!


#6

Intermittent problems often don’t show up when things are being “checked.” A new vacuum line and check valve for the booster are cheap. Start there. But do note the pedal feel next time.

The connection of the surging, btw, is that a malfunctioning booster, line, or valve can make a vacuum leak. Vacuum leaks make an engine surge.